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1. Explain on the "fast" query comes up with 2 rows:
On the users table (M), it ends up using a "where" clause and looking at 153,000 records. On the subscribers table (N), it ends up looking at 1 row using "where, not exists" but also relying on the "user_id" index of "N".

2. Explain on the "slow" query comes up with 2 rows:
On the users table (M), it ends up using a "where" clause and looking at 153,000 records. On the subscribers table (N), it ends up using a "where, not exists" clause and looking at 152,000 records without using an index, this is probably a big part of the problem.

3. Storage engine for both tables is: MyISAM.

4. The fast query takes about 3 to 4 seconds, the slow query is over 50 seconds and essentially holds up the dedicated server we are using.

Comment

Each table has an email address unique index, field type is varchar(100). I'm including the table information for both below. As I'm writing this, I noticed that the charset of users is set as utf8 and the charset of subscribers is set as latin1. Would that matter?